At the outbreak of World War II, Crabb was first an army gunner. Then, in 1941, he joined the Royal Navy. The next year he was sent to Gibraltar where he worked in a mine and bomb disposal unit to remove the Italian limpet mines that enemy divers had attached to the hulls of Allied ships. Initially, Crabb's job was to disarm mines that British divers removed, but eventually he decided to learn to dive.

On 8 December 1942, during one such attack, two of the Italian frogmen, Lt. Visintini and Petty Officer Magro died, probably killed by depth charges. Their bodies were recovered, and their swimfins and Scuba sets were taken and from then on used by Sydney Knowles and Commander Lionel Crabb.

By this time he had gained the nickname "Buster", named after U.S. actor and swimmer Buster Crabbe. After the war Crabb was stationed in Palestine and led an underwater explosives disposal team that removed mines placed by Irgun, the Zionist militant group. After 1947, he was demobilised from the military.

Crabb moved to a civilian job and used his diving skills to explore the wreck of a Spanish galleon and he located a suitable site for a discharge pipe for the Atomic Weapons Research Establishment at Aldermaston. He later returned to work for the Royal Navy. He twice dived to investigate sunken Royal Navy submarines — the HMS Truculent in January 1950 and HMS Affray in 1951 — to find out whether there were any survivors. Both efforts were fruitless. In 1952 Crabb married Margaret Elaine Player, the daughter of Henry Charles Brackenbury Williamson and the former wife of Ernest Albert Player. The couple separated in 1953 and divorced about two years later.[citation needed]

In 1955 Crabb took frogman Sydney Knowles with him to investigate the hull of the Soviet cruiser Sverdlov to evaluate its superior manoeuvrability. According to Knowles, they found a circular opening at the ship's bow and inside it a large propeller that could be directed to give thrust to the bow. That same year, March 1955, Crabb was made to retire due to his age, but a year later he was recruited by MI6. By this point, Crabb's heavy drinking and smoking had taken its toll on his health, and Crabb was not the diver that he had been in World War II.[2]

MI6 recruited Crabb in 1956 to investigate the Soviet cruiser Ordzhonikidze that had taken Nikita Khrushchev and Nikolai Bulganin on a diplomatic mission to Britain. According to Peter Wright in his book Spycatcher (1987), Crabb was sent to investigate the Ordzhonikidze's propeller — a new design that Naval Intelligence wanted to examine. On 19 April 1956 Crabb dived into Portsmouth Harbour and his MI6 controller never saw him again. Crabb's companion in the Sally Port Hotel took all his belongings and even the page of the hotel register where they had written their names. Ten days later British newspapers published stories about Crabb's disappearance in an underwater mission.

MI6 tried to cover up this espionage mission. On 29 April, under instructions from Rear Admiral J.G.T. Inglis OBE, the Director of Naval Intelligence,[2] the Admiralty announced that Crabb had vanished when he had taken part in trials of secret underwater apparatus in Stokes Bay on the Solent. The Soviets answered by releasing a statement stating that the crew of the Ordzhonikidze had seen a frogman near the cruiser on 19 April.

British newspapers speculated that the Soviets had captured Crabb and taken him to the Soviet Union. The British Prime MinisterAnthony Eden apparently disapproved of the fact that MI6 had operated without his consent in the UK (the preserve of the Security Service, "MI5"). It is mistakenly claimed that Eden forced director-general John Sinclair to resign following the incident. In fact, he had determined to replace Sinclair with MI5 director-general Dick White before the incident.[3] Eden told MPs it was not in the public interest to disclose the circumstances in which the frogman met his end.[4][5]

A little less than 14 months after Crabb's disappearance, on 9 June 1957, a body in a frogman suit was found floating off Pilsey Island in Chichester Harbour. It was missing its head and both hands, which made it impossible to identify (using then-available technology).[2] The body had the same height as Crabb, the same body-hair color, and was dressed in the same clothes, Pirelli two-piece diving suit and Admiralty Pattern swim fins that Crabb was wearing when he embarked on his final mission.[2] The British diving expert Rob Hoole wrote that given the length of time that Crabb's body had been in the water, there was "nothing sinister" about the missing head and hands.[2] Crabb's ex-wife was not sure enough to identify the body, nor was Crabb's girlfriend Pat Rose, though Sydney Knowles said that Crabb had had a similar scar on the left knee. An inquest jury returned an open verdict but the coroner announced that he was satisfied that the body was that of Lionel Crabb.

As information was declassified under the 50-year rule new facts on Crabb's disappearance came to light. On 27 October 2006, the National Archives released papers relating to the fatal Ordzhonikidze mission.[6] Sydney Knowles, a former diving partner of Crabb's, stated on televised interview on Inside Out - South: 19 January 2007 for the BBC that Crabb didn't dive alone on his fatal last mission

“

He told me they'd given him a buddy diver.

”

Furthermore papers released under the Freedom of Information Act indicate that there were other divers investigating the Ordzhonkidze while she was in Portsmouth Harbour.[7] On 9 November 2007, The Independent reported how the government had covered up the death of 'Buster' Crabb.[8]

The cruiser Ordzhonikidze was later transferred by the Soviet government to Indonesia in 1962, where it operated as KRI Irian. The ship operated in the conflict against the Netherlands over West Papua, and was later used as floating detention centre for suspected communists during the Indonesian killings of 1965–1966. The cruiser was scrapped in 1971.

The spy Harry Houghton wrote a book called "Operation Portland" after he was released from prison in which he outlined the explanation of Crabb's death which he claimed to have been given by his Russian handler, a man he knew as Roman, in July 1956. Houghton claims that shortly before the Soviet visit he had been meeting Roman in a pub in Puncknowle, Dorset, and happened to see a friend who worked at the Underwater Detection Establishment with her boyfriend who was a diver. The boyfriend was annoyed that he had been training for something special, which had just been called off. Shortly after hearing this, Roman had cut short the meeting.[9]

According to this account, after guessing that there may be some attempt by divers connected with the Ordzhonikidze, the Soviet Navy had arranged for six underwater sentries to watch the bottom of the ship, which had been fitted with wire jackstays on either side to help them hold on to. When Crabb arrived, a struggle ensued in which Crabb's oxygen supply was turned off and he passed out. He was then hauled on board, and taken to the sick bay (having passed out a second time) and given medical treatment.[10]

When Crabb had recovered sufficiently, the Soviets began to interrogate him; Crabb was making a confession when he collapsed and this time did not recover. The Soviets, aware that they might be accused of causing his death, decided to fix his body lightly to the bottom of the ship so that it came loose once the ship was under way. In the event, the body entangled in something underwater which meant it did not get discovered for fourteen months. Houghton also puts forward the theory that Crabb's mission was to plant a small limpet mine on the Ordzhonikidze whose purpose was to detect whether the Soviet Navy was using the latest sonar technology: if it was, the mine would detonate, and the ship would slow down; if not, the mine would eventually detach and go to the bottom of the sea.[11]

In a 1990 interview Joseph Zwerkin, a former member of Soviet Naval intelligence who had moved to Israel after the breakup of the Soviet Union, claimed that the Soviets had noticed Crabb in the water and that a Soviet sniper had shot him. Official government documents regarding Crabb's disappearance are not scheduled to be released until 2057.[12]

On 16 November 2007, the BBC and the Daily Mirror reported that Eduard Koltsov, a Sovietfrogman, claimed to have caught Crabb placing a mine on the Ordzhonikidze hull near the ammunition depot and cut his throat. In an interview for a documentary film, Koltsov showed the dagger he allegedly used in a Russian documentary as well as an Order of the Red Star medal that he claimed to have been awarded for the deed.[4][13] Koltsov, 74 at the time of the interview, stated that he wanted to clear his conscience and make known exactly what happened to Crabb.[14] It seems extremely unlikely that the British government would have tried to blow up a Soviet ship on a diplomatic mission while it was anchored in British waters carrying the leaders of the Soviet Union, making Koltsov's claim of a mine suspect.[14] A Russian journalist from the military Krasnaya Zvezda newspaper considers Koltsov's story improbable. In particular the archive documents do not confirm that Koltsov (a city bus driver by 30 years in Rostov-on-Don) was awarded Order of the Red Star or was a Soviet frogman.[15]

Certain Members of Parliament and Michael Hall became concerned about Crabb's ultimate fate and in 1961, Commander J.S. Kerans (and later in 1964 Marcus Lipton) submitted proposals to re-open the case but were rebuffed. Various people speculated that Crabb had been killed by some secret Soviet underwater weapon; that he had been captured and imprisoned in Lefortovo prison with prison number 147, that he had been brainwashed to work for the Soviet Union to train their frogman teams; that he had defected and became a commander in the Soviet Navy;[16] that he was in the Soviet Special Task Underwater Operational Command in the Black Sea Fleet; or that MI6 had asked him to defect so he could become a double agent.[2]

On 26 March 2006, the Mail on Sunday published an article by Tim Binding entitled "Buster Crabb was murdered – by MI5". Binding wrote a fictionalised account of Crabb's life, Man Overboard which was published by Picador in 2005.[17] Binding stated that, following the book's publication, he was contacted by Sydney Knowles.[18] Binding alleged that he then met Knowles in Spain and was told that Crabb was known by MI5 to have intentions of defecting to the USSR. This would have been embarrassing for the UK — Crabb being an acknowledged war hero. Knowles has suggested that MI5 set up the mission to the Ordzhonikidze specifically to murder Crabb, and supplied Crabb with a new diving partner who was under orders to kill him. Binding stated Knowles alleged that he was ordered by MI5 to identify the body found as Crabb, when he knew it was definitely not Crabb. Knowles went along with the deception. Knowles has also alleged that his life was threatened in Torremolinos in 1989, at a time when Knowles was in discussions with a biographer. About the claims that Crabb was planning to defect to the Soviet Union, Reg Vallintine of the Historical Diving Society was quoted as saying:"Diving historians find it very hard to believe that this man, who prided himself on being a patriot, would have seriously considered defecting. Crabb was very fond of being a hero, and it is hard to imagine him jeopardising that status."[18] It is not clear just why MI6 would recruit a man who was known to be planning to defect to the Soviet Union to spy against the Soviet Union or why Crabb would agree to such a mission if he really had decided that he wanted to live in the Soviet Union.

The British diving expert Rob Hoole wrote in 2007 that Crabb had probably died of oxygen poisoning or perhaps carbon dioxide poisoning, and that Crabb's age and poor health caused by his heavy drinking and smoking had made him unsuitable for the mission that he had been assigned.[2] In support of the death by misadventure theory, Hoole noted that before disappearing on his second attempt to dive the Ordzhonikidze, Crabb had during his first attempt experienced equipment failure, which suggested that Crabb's equipment was not up to standard.[2] Crabb's MI6 officer John Nicholas Rede Elliott always took the view that Crabb had suffered equipment failure and/or his health had given way, and that his reputation had been unfairly dragged through the mud.[19]

The 1996 novel Old Flames by John Lawton concerns the murder of a fictitious frogman with a secret life.[20]

Tim Binding's 2005 novel Man Overboard is a fictional memoir of Crabb, who looks back over his career from a sanatorium in Czechoslovakia, having been seized by the KGB on his final mission for the British.

John Ainsworth Davis/Christopher Creighton in his thinly disguised 1987 fictional account The Krushchev Objective with co-author Noel Hynd, states he was the second diver with Crabb that thwarted an assassination attempt on the Soviet dictator by defusing limpet mines.